(a) Field of the Invention
This invention relates generally to mobile satellite communications systems and, more particularly, to a service area modeling system that supports geographically defined satellite communications service and a method of using the modeling system for rapid identification of the service area in which a mobile user is located.
(b) Description of Related Art
In terrestrial cellular communications systems, a service area is defined as a geographical region within which a communications service provider has made available particular mobile calling services and charges particular billing rates. All mobile callers located within a given service area are treated equally in terms of billing rates (according to subscription) and available services, so it is necessary to determine the service area within which a mobile caller is located in order to be able to properly process a call initiated by the caller.
In the terrestrial cellular communications environment, each stationary cellular tower provides communication services to a geographic region referred to as a cell which is typically about one or two kilometers in diameter and each tower transmits calls at a range of frequencies allocated exclusively to the tower. In addition, each tower is exclusively associated with the service area in which the tower is geographically located such that each tower provides services to the cell in accordance with the service area in which the tower is located. Because each tower transmits at a unique range of frequencies, the transmission frequency of a call may be used to identify the tower that is transmitting the call and, due to the one-to-one correspondence between towers and service areas, once the tower transmitting the call has been identified, the service area is also known. Thereafter, an identifier (xe2x80x9cIDxe2x80x9d) corresponding to the identified service area is transmitted to a switch that connects the cellular communications network to the public switched telephone network. The switch is programmed to provide the services and billing rates defined for that service area ID so that the call may be properly processed.
This system of call processing in which the call transmission frequency is used to identify the transmitting tower which is then used to identify the service area in which the user is located so that the call may processed accordingly works well in the terrestrial cellular communications environment, in part, because the towers are stationary and the cells (transmitting ranges of towers) are relatively small in size. However, this same call processing system is not suited for the satellite communications environment. In particular, in the satellite environment, the counterpart to the stationary towers of the cellular environment include satellite mounted spotbeam antennas. These spotbeam antennas project ago spotbeams onto the earth and provide communication service to all system users located within the area onto which the spotbeam projects. Thus, the spotbeam may be considered, in some ways, the satellite environment equivalent to the cell of the cellular environment. However, spotbeams project onto geographic regions that are typically on the order of hundreds of kilometers, and, therefore, may encompass a plurality of service areas. As a result, there is not a one-to-one correspondence between spotbeams and service areas. Therefore, in the satellite environment, knowing the identity of the satellite spotbeam transmitting the communications service, e.g., the call, does not enable the identification of a single service area in the cellular environment. In addition, in a non-geo stationary satellite system, low to medium earth orbit satellites are orbiting the earth at rapid speeds so that the spotbeams projected onto the earth""s surface by the satellite are also in motion. Thus, the geographic region onto which a satellite cell is projected is changing as the satellite moves, thereby making it impossible to correlate a spotbeam with a single geographically defined service area.
Further, although some satellite systems are equipped to approximate the location of a subscriber unit, the approximation is typically subject to error. This error may span a geographical region that overlaps several service areas and thus is not useful in pinpointing the service area in which the mobile caller is located. In addition, service areas typically have complex geometric shapes as they are often configured to coincide with national or other geopolitical borders so that this error is even less tolerable when the caller is located near an intricate border.
In systems in which the longitudinal and latitudinal coordinates of the subscriber unit can be determined with a satisfactory degree of accuracy, the system could potentially determine the service area of the subscriber unit by identifying the service area corresponding to the coordinate position of the mobile subscriber. However, such a method would require a table in which each of the infinite number of coordinate points on the surface of the earth are associated with a service area. Such an infinitely long list cannot be compiled and would, at any rate, be inefficient and time consuming to search.
Another difficulty is encountered when attempting to model service areas in the satellite communications environment. In particular, geographically defined service areas modeled on the surface of the earth in the satellite environment are three dimensional due to the quasi-spherical shape of the earth. As a result, complex and time consuming three dimensional geometry is required to determine the location of the caller relative to the three dimensional service areas.
Although geographically defined service areas are not an inherent property of mobile satellite communications systems, they are well established in the terrestrial cellular communications system. As a result, existing terrestrial architecture includes equipment adapted to process calls using the geographically defined service areas. In particular, once the service area has been identified for an on-going mobile telephone call, an ID corresponding to the identified service area is transmitted to a mobile switching center. In response, the switch, which has been programmed with the services and billing rates to be provided for the service area, processes the call accordingly. The mobile switching centers used to perform this service area-dependent call processing are complex and costly. Therefore, it is desirable that service areas in a satellite communications system be defined on a regional basis so that the mobile switching centers and other equipment used to process calls in the terrestrial cellular invention may also be used to process calls originating from the satellite environment with little modification.
Thus, there is a need for an improved method of determining, in a satellite communications system, the service area within which a mobile user is located. Preferably, such a method would be efficient, rapid and compatible with the existing geographically defined service area system and associated equipment used in terrestrial cellular communications systems.
The present invention is embodied in a method of mapping a plurality of geographic regions on the surface of the earth, using the mapping to determine a target geographic region within which a target is located and then providing a service to the target that is associated with the target geographic region. Preferably, the disclosed method may be implemented in a satellite communication system to provide a satellite communication service to a subscriber unit located in a geographical region where a set of previously determined services are readily available such as a particular service area. The method is efficient, rapid and compatible with existing geographically defined service areas systems and associated equipment used in terrestrial cellular communication systems.
In accordance with one aspect of the invention, a method for mapping a plurality of geographical regions on the surface of the earth is provided wherein the method includes the steps of dividing each of the geographical regions into a plurality of non-overlapping tiles such that each tile is associated with only one geographical region and has four edges. Once defined, the tiles are mapped onto a two dimensional plane that represents the surface of the earth and a grid having a plurality of grid squares is overlaid onto the two dimensional plane.
In one aspect of the invention, the grid has both vertical grid lines and horizontal grid lines, wherein the vertical grid lines align with the lines of longitude and wherein the horizontal grid lines align with the lines of latitude. The vertical grid lines and the horizontal grid lines intersecting to form the plurality of grid squares.
In another aspect of the invention the method may be used to determine the target geographical location within which a target is located, wherein the target is positioned at a target location, using the map of the geographical regions and also using a previously determined estimate of the target location. First, the two dimensional plane is used to determine a target grid square in which the estimate of the target location is located. Next, a first set of tiles that intersect the target grid square are identified. Then a first set of geographical regions are obtained wherein the first set of geographical regions includes all of the regions associated with the tiles in the first set of tiles.
In a preferred embodiment, the first set of tiles are identified by creating a table that includes a list of each of the grid squares, wherein each of the grid squares corresponds to a different one of a plurality of lists, and wherein each of the lists comprises all tiles that overlap the corresponding grid square. Then the table may be referenced to obtain the unique list of tiles corresponding to the target grid square, wherein the unique list of tiles is subsequently used as the first set of tiles.
In another aspect of the invention, the method further includes the steps of determining whether the first set of geographical regions comprises a single one of the geographical regions; and if the first set of geographical regions comprises a single one of the geographical regions, then identifying the single one of the geographical regions as the target geographical region. If the first set of geographical regions does not comprise a single one of the geographical regions, then a target tile is identified, wherein the target tile comprises a tile in the first set of tiles within which the estimate of the target location in located. Next, the geographical region that is associated with the target tile is identified as the target geographical region. Lastly, a service that is associated with the target geographical region is provided to the target.
In one embodiment, the method may be employed in the context of satellite communications wherein each of the geographical regions comprises a service area, and each service area is associated with a satellite communication service. In this setting, the target may be a subscriber unit and the service that is provided to the subscriber unit is a satellite communication service.
In yet another aspect of the present invention, the method may be used to determine the target geographical region within which a target is located, using a previously determined estimate of the target location and using the map wherein the previously determined estimate of the target location comprises a region of error. The method includes the steps of identifying at least one target grid square, that intersects, at least in part, with the error region. Then a first set of tiles that intersect the target grid square are identified and used to obtain a set of geographical regions that are associated with the tiles in the first set of tiles.
In one aspect of the invention, the step of identifying the target grid square may be accomplished by converting the error region from a three dimensional region to a two dimensional region, overlaying the converted error region on the two dimensional plane and identifying the target grid square that intersects with the error region.
In one aspect the error region comprises a circular error region centered at the estimate of the target location such that the step of converting the region to a two dimensional region causes the region to become an elliptical region, and wherein the elliptical region is represented as a region bounded by a set of four boundaries that forming a rectangular region encompassing the elliptical region.
In yet another aspect of the invention, the method checks to see if the first set of geographical regions includes only a single one of the geographical regions. If so, then the single geographical region is the region within which the target geographical region and the service associated with the region is provided. If, instead, the first set of geographical regions does not include only one geographical region, then a second set of tiles are obtained that includes the tiles in the first set of tiles that intersect a boundary of the error region.
In one aspect of the invention, the step of obtaining the second set of tiles includes the steps of using geometrical methods, which may include point in polygon inclusion methods or ellipse and polygon intersection test, to determine whether any of the edges of the tiles in the first set of tiles intersect the error region boundary; and adding the tiles having an edge that intersects the error region boundary to the second set of tiles.
Lastly, a second set of geographical regions are identified wherein the second set of geographical regions includes all of the geographical regions that are associated with each of the tiles listed in the second set of tiles and then the method tests to determine whether the second set of geographical regions includes only a single one of the geographical regions. If so, then the one geographical region is identified as the target geographical region and service associated with that region is provided to the target. If, instead, the second set comprises more than one of the geographical regions, then a virtual geographical region is identified as the target geographical region and a service associated with the virtual geographical region is provided to the target.
The present invention further includes an apparatus coupled to a communication network for providing a communication service to a target located at a target location, wherein an estimate of the target location has been previously determined, the apparatus includes a switch coupled to the communication network, and a processor coupled to the switch. The processor is programmed to determine the target geographical location within which the target is located using the predetermined estimate of the target location and a map, wherein the map includes a plurality of geographical regions, wherein each of the geographical regions has been divided into a plurality of tiles such that each tile is associated with only one geographical region and has four edges. Additionally, the tiles have been mapped onto a two dimensional plane; wherein a grid having a plurality of grid squares has been overlaid on the two dimensional plane. The apparatus additionally includes a memory coupled to the processor, wherein data representing the map is stored in the memory. The processor is further adapted to provide the target geographical region to the switch which, in turn, is adapted to provide a communication service to the target based on the target geographical region provided by the processor.
In yet another aspect of the present invention, the apparatus further includes a central management computer coupled to the processor, wherein the central management computer is coupled to a graphical user interface that is adapted to create the map. The map is subsequently provided by the central management computer to the processor.